Incandescent tension annealing processes for strong, twist-stable carbon nanotube yarns and muscles

ABSTRACT

The described incandescent tension annealing processes involve thermally annealing twisted or coiled carbon nanotube (CNT) yarns at high-temperatures (1000° C. to 3000° C.) while these yarns are under tensile loads. These processes can be used for increasing yarn modulus and strength and for stabilizing both twisted and coiled CNT yarns with respect to unwanted irreversible untwist, thereby avoiding the need to tether torsional and tensile artificial muscles, and increasing the mechanical loads that can be moved by these muscles.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to provisional U.S. Patent ApplicationSer. No. 62/328,242, filed Apr. 27, 2016, entitled “Incandescent TensionAnnealing Processes For Strong, Twist-Stable Carbon Nanotube Yarns AndMuscles,” which provisional patent application is commonly owned by theApplicant of the present invention and is hereby incorporated herein byreference in its entirety for all purposes.

GOVERNMENT INTEREST

This invention was made with government support under the Air ForceOffice of Scientific Research grants FA9550-12-1-0035, FA9550-12-1-0211,and FA2386-13-1-4119; Robert A. Welch Foundation grant AT-0029; NationalScience Foundation grant CMMI1335204; Office of Naval Research MURIgrant NOOD14-11-1-0691; and the Army grant W91CBR-13-C-0037. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is directed to incandescent tension annealingprocesses for strong, twist-stable carbon nanotube yarns and muscles.

BACKGROUND

Twist-spun carbon nanotube (CNT) yarns are of great interest for suchdiverse applications as artificial muscles [Foroughi 2011; Lima 2012; P.Chen 2015], supercapacitors [X. Chen 2013], batteries [Weng 2014], andintelligent textiles and structural composites [Lu 2012; Liu 2010].While inserted twist can generate new properties [Foroughi 2011] andimprove other properties [M. Zhang 2004], an important problem exists:single-ply twisted or coiled neat CNT yarns will irreversibly untwistunless they are torsionally tethered [Li 2013]. This problem isparticularly troublesome for twist-spun CNT yarn artificial muscles,which can be driven either electrochemically [Foroughi 2011],electrothermally [Lima 2012], or chemically [Lima 2015] to providetorsional and tensile actuation. It is also a key problem for twistretention during weaving CNT yarns.

Various important means are now available for continuously making CNTyarns by either liquid-state [Vigolo 2000; Ericson 2004; Behabtu 2013]or dry-state methods [M. Zhang 2014; Jiang 2002; Li 2004; X. F. Zhang2007; X. B. Zhang 2006; Jayasinghe 2011].

The present invention is directed to yarns made by a twist-basedprocess, wherein CNT aerogel sheets drawn from spinnable nanotubeforests are twisted into yarn during the draw process [M. Zhang 2004].Twist-spun yarns have importance in providing high-performance torsionaland tensile artificial muscles [Foroughi 2011; Lima 2012; P. Chen 2015],which are called either twisted yarns or coiled yarns, depending uponwhether the inserted twist is below or above the amount required toproduce yarn coiling. When infiltrated with electrolyte andelectrochemically driven, these two-end tethered yarns can rotate arotor at speeds exceeding 590 rpm, providing torsional strokes per yarnlength of 125° mm⁻¹ [Foroughi 2011]. Infiltration with volume-changingguests produced twisted yarns that provided torsional speeds of up to11,500 rpm [Lima 2012]. The coiled yarns infiltrated with paraffin waxand silicone rubber could be thermally [Lima 2012] or chemically [Lima2015] actuated to accomplish 0.836 kJ kg⁻¹ and 1.2 kJ kg⁻¹ of mechanicalwork during muscle contraction, respectively. These work capacities aregreater than 31 times that for natural muscle (0.039 kJ kg⁻¹) [Josephson1993].

Despite the impressive performance, single-ply twisted and single-plycoiled CNT yarn muscles must be torsionally tethered to preventirreversible untwist during tensile actuation [Lima 2012] and need anon-actuating segment or the infiltration of elastic guest materials asa returning spring for torsional actuation [Foroughi 2011; Lima 2012;Lima 2015], which could cause inconveniences for practical applications.The tensile work capacity of these muscles increases with increasingload until the yarn muscle mechanically fails [Lima 2015]. The presentinvention provides increased mechanical bonding within the yarnstructure that increases both twist retention and mechanical strength.

While infiltration of CNT yarns with polymers provides a well-knownmeans to increase yarn strength, modulus and toughness [Liu 2010; Fang2010; Ryu 2011], such infiltration cannot be generically applied for CNTyarn muscles, since volume changes of electrolyte or guest within theyarn drive the actuation of the yarn muscle. An alternative approach isto covalently link adjacent nanotubes, such as by using radiation [Kris2004; Krasheninnikov 2007; Filleter 2003]. Irradiating carbondouble-walled nanotube (DWNT) bundles by an electron beam in an electrontransmission microscope increased the tensile strength and elasticmodulus of the individual nanotube bundle by an order of magnitude, upto maximum values of 1.5-17.1 GPa and 103-693 GPa, respectively[Filleter 2011]. However, application of this approach tomicrometers-thick CNT yarns is practically limited by the shortpenetration length of highly absorbed electron beams. Irradiation of CNTyarns by gamma rays in air increased strength and modulus of CNT yarnspossibly due to the formation of carboxyl like groups between adjacentnanotubes, but the final tensile strength of these irradiated yarns wasonly about 850 MPa [Miao 2011]. Fan's team has importantly shown thatthermally annealing twisted CNT yarns in vacuum for several hours at2000 K, without significant applied tensile stress, increased Young'smodulus from 37 to 74 GPa, but slightly decreased yarn strength (from600 to 564 MPa) [X. B. Zhang 2006].

SUMMARY OF INVENTION

Embodiments of the present invention provide a process for stabilizingboth twisted and coiled CNT yarns with respect to unwanted irreversibleuntwist, thereby avoiding the need to tether torsional and tensileartificial muscles, and increasing the mechanical loads that can bemoved by these muscles. This process is called ITAP, which is anabbreviation for “Incandescent Tension Anneal Process”, since thisprocess involves thermally annealing a carbon nanotube yarn atincandescent temperatures while the yarn is subjected to tensile stress.

In general, the invention features applying an incandescent tensionannealing process to a CNT yarn. In one aspect, the process includes:

-   -   a. Wrapping a CNT yarn around two molybdenum hook electrodes.    -   b. Applying an electrical current through the electrodes to heat        the yarn (or an assembly of parallel yarns) to 1000 C-3000 C in        a vacuum.    -   c. Before incandescently heating the yarns, applying a small        current to remove the oxygen adsorbed on the CNTs.    -   d. Applying tensile stress by hanging various size weights on        the CNT yarns through the bottom molybdenum hook electrode        during high-temperature annealing, while the yarn is torsionally        tethered to largely prohibit yarn untwist. The maximum applied        stress being about 45% of the fracture strength of the twisted        pristine CNT yarns.    -   e. After interruption of the current at the end of annealing,        cooling the yarns to room temperature in the vacuum.

The presently described incandescent tension annealing process could beapplied to CNT yarns spun from CNT forests, CNT solution, and CNTs grownby floating catalytic chemical vapor deposition.

The presently described incandescent tension annealing process could beapplied to CNT composite yarns comprising guest material, such asgraphene oxide, graphene, or ceramics. Such composite yarns can beoptionally made by a biscrolling process, wherein the guest material isdeposited on a carbon nanotube sheet before or during twist insertion tomake a twisted yarn or a coiled yarn [S. Fang '375 PCT Application].

The presently described incandescent tension annealing process can beconducted in a vacuum or in inert gases such as nitrogen, helium, andargon.

The presently described incandescent tension annealing process includesapplying tension on the CNT yarns during high-temperature annealingprocess. The tension can be applied, for instance, by hanging weights onCNT yarns or using tension rods during the continuous processing of CNTyarns.

The presently described incandescent tension annealing process includesapplying tension on the CNT yarns at high temperatures of 1000-3000 C,while the yarn is torsionally tethered to largely prohibit yarn untwist.The high annealing temperatures can be achieved by applying currentthrough a CNT yarns, placing the yarn in a high temperature environment,heating the yarn by the absorption of electromagnetic radiation,inductively heating of the yarn, or by a combination of these heatingmethods.

The presently described incandescent tension annealing process can beconducted continuously or batch by batch.

In general, in another aspect, the invention features a process thatincludes the step of applying a tensile stress to a CNT yarn. Theprocess further includes the step of high-temperature annealing the CNTyarn while the tensile stress is applied to the CNT yarn to form an ITAPyarn. The high-temperature annealing is performed in the temperaturerange between 1000 C and 3000 C. The ITAP yarn has a characteristicselected from the group consisting of (i) the tensile strength of theITAP yarn is greater than the pristine CNT yarn, (ii) the tensilemodulus of the ITAP yarn is greater than the pristine CNT yarn, (iii)the pristine CNT yarn was a twisted or coiled CNT yarn, and the twistedor coiled ITAP yarn is stabilized with respect to irreversible untwistor snarling, thereby avoiding the need to tether the twisted or coiledITAP yarn, (iv) the ITAP yarn is stabilized with respect tochemically-induced yarn degradation, and (v) combinations thereof.

Implementations of the inventions can include one or more of thefollowing features:

The step of high-temperature annealing can include heating the yarn by amethod selected from the group consisting of: (a) applying an electricalcurrent through the CNT yarn, (b) placing the yarn in a high-temperatureenvironment, (c) absorption of electromagnetic radiation, (d) inductiveheating, and (e) combinations thereof.

The CNT yarn can be not coiled.

The CNT yarn can be twisted. The tensile stress applied to the CNT yarnduring the step of high-temperature annealing can be at least 5% offracture strength of the CNT yarn at room temperature before the step ofhigh-temperature annealing.

The tensile stress applied to the CNT yarn during the step ofhigh-temperature annealing can be at least 20% of the fracture strengthof the CNT yarn at room temperature before the step of high-temperatureannealing.

The tensile stress applied to the CNT yarn during the step ofhigh-temperature annealing can increase with increasing time over afirst time period occurring during the step of high-temperatureannealing, while maintaining the tensile stress at below an appliedstress level that would cause damage to the CNT yarn at thehigh-temperature annealing temperature.

The CNT yarn can be coiled and mandrel-free. The tensile stress appliedto the CNT yarn can be in an amount that avoids yarn snarling of the CNTyarn.

The tensile stress applied to the coiled CNT yarn can be at least 1% offracture strength of the coiled CNT yarn at room temperature before thestep of high-temperature annealing.

An inert environment can be employed during the step of high-temperatureannealing.

The process can further include removing oxygen adsorbed from the CNTyarn before the CNT yarn reaches incandescent temperatures duringannealing.

The step of removing oxygen can include a method selected from the groupconsisting of: (a) applying an electrical current through the CNT yarn,(b) placing the yarn in a high-temperature environment, (c) absorptionof electromagnetic radiation, (d) inductive heating, and (e)combinations thereof.

The time of the step of high-temperature annealing can range from 0.1milliseconds to 2 hours.

The ITAP treatment time can be shortened by increasing the temperatureat which the ITAP treatment is accomplished.

The process can further include the method of forming the CNT yarn. Thestep of forming the CNT yarn can be selected from the group consistingof spinning from CNT forests, CNT solutions, and CNT aerogel sheetsgrown by floating catalytic chemical vapor deposition.

The process can be a continuous process or a batch by batch process.

The CNT yarn can be part of an assembly of CNT yarns.

The CNT yarns in the assembly can be held under different levels oftensile stresses and temperatures.

Not all of the CNT yarns in the assembly can be subjected to the processdescribed above.

All of the CNT yarns in the assembly can be substantially subjected tothe same stress and temperature.

The CNT yarns can be woven into a textile.

At least some portion of the CNT yarns can be plied.

Segments of the CNT yarn can be subjected to the process described aboveand other segments of the CNT yarn cannot be subjected to the processdescribed above.

The CNT yarn can further include at least one additional material otherthan CNTs.

The CNT yarn can include a ceramic material.

The CNT yarn can include a second carbon material. The second carbonmaterial is not CNTs.

The second carbon material can include graphene or a graphenederivative.

The CNT yarn can substantially include only CNTs.

In general, in another aspect, the invention features a coiled or highlytwisted CNT yarn that substantially contains twist in only one directionand comprises substantially only CNTs. The CNT yarn has a characteristicselected from the group consisting of (i) the CNT yarn does not undergosnarling when untethered, (ii) the CNT yarn substantially retains twistduring the release of tethering even when snarling is prohibited, (iii)the CNT yarn substantially retains mechanical strength when exposed tochlorosulfonic acid for 5 minutes at ambient temperature, and (iv)combinations thereof.

Implementations of the inventions can include one or more of thefollowing features: The CNT yarn can include each of the followingcharacteristics (i) the CNT yarn does not undergo snarling whenuntethered, (ii) the CNT yarn substantially retains twist during therelease of tethering even when snarling is prohibited, and (iii) the CNTyarn substantially retains mechanical strength when exposed tochlorosulfonic acid for 5 minutes at ambient temperature.

In general, in another aspect, the invention features an artificialmuscle, composite structure, or textile including one or more coiled orhighly twisted CNT yarns made by a process that includes applying atensile stress to a CNT yarn that substantially includes only CNTs. Theprocess to make the one or more coiled or highly twisted CNT yarnsfurther includes high-temperature annealing the CNT yarn while a tensilestress is applied to the CNT yarn to form an ITAP yarn. Thehigh-temperature annealing is performed in the range between 1000 C and3000 C.

Implementations of the inventions can include one or more of thefollowing features:

The artificial muscle, composite structure, or textile can include oneor more twisted CNT yarns. The tensile stress applied to the CNT yarnsduring the step of high-temperature annealing can be at least 10% offracture strength of the CNT yarn at room temperature before the step ofhigh-temperature annealing.

The artificial muscle, composite structure, or textile can include oneor more twisted CNT yarns. The tensile stress applied to the CNT yarnsduring the step of high-temperature annealing can be at least 1% of thefracture strength of the coiled CNT yarn at room temperature before thestep of high-temperature annealing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a setup used for an incandescent tension anneal process ofthe present invention.

FIG. 2A is a top-view SEM image of a pristine yarn.

FIG. 2B is a top-view SEM image of an ITAP-40 yarn.

FIG. 2C is a cross-sectional SEM image of the pristine yarn of FIG. 2A.

FIG. 2D is a cross-sectional SEM image of the ITAP-40 yarn of FIG. 2B.

FIG. 3 is a graph showing comparisons of the specific strength andspecific modulus of a pristine yarn (1.08 g cm⁻³ in density) and ofcorresponding ITAP yarns annealed under different applied stresses,where the applied stress during ITAP (σ), is normalized to theroom-temperature fracture strength of the pristine yarn (σ_(max)).

FIG. 4A is a graph showing specific strength as a function of annealingtime (at 2000° C. under 30% σ_(max)) during the ITAP.

FIG. 4B is a graph showing specific modulus as a function of annealingtime (at 2000° C. under 30% σ_(max)) during the ITAP.

FIG. 5A is an SEM image of a pristine coiled multiwalled carbon nanotube(MWNT) yarn that has snarled during untethering.

FIG. 5B is an SEM image of the same type of pristine coiled MWNT yarn ofFIG. 5A that was untethered after ITAP-1.5.

FIG. 6 is a graph showing comparisons of specific torque generated in apristine yarn, t(pristine), and in an ITAP-40 yarn, t(ITAP), as a resultof applying a tensile stress.

FIG. 7A is a graph showing comparisons of stress-strain curves forpristine yarns and ITAP-25 yarns before and after treatment inchlorosulfonic acid for 5 minutes and subsequent removal of thischlosulfonic acid from the yarn.

FIG. 7B is a photograph of the pristine yarns and ITAP-25 yarns beforeimmersion in chlorosulfonic acid for 5 minutes.

FIG. 7C is a photograph of the pristine yarns and ITAP-25 yarns afterimmersion in chlorosulfonic acid for 5 minutes.

DESCRIPTION OF THE INVENTION

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings, which form a part hereof, andwhich show, by way of illustration, specific example embodiments.Subject matter may, however, be embodied in a variety of different formsand, therefore, covered or claimed subject matter is intended to beconstrued as not being limited to any example embodiments set forthherein; example embodiments are provided merely to be illustrative.Likewise, a reasonably broad scope for claimed or covered subject matteris intended. Among other things, for example, subject matter may beembodied as methods, devices, components, or systems.

As embodied and broadly described, this invention is directed to themethod of incandescent tension anneal process for strong, twist-stablecarbon nanotube yarns and muscles.

In the present invention, a unique method of incandescent tension annealprocess, described above, is used for preparing strong, twist-stablecarbon nanotube yarns and muscles. FIG. 1 shows the setup 100 used forthe incandescent tension anneal process that includes a carbon nanotubeyarn 101 wrapped around (or otherwise connected) to electrodes 102 and103, a weight 104 (attached to electrode 103, which is shown in theshape of a hook, such as a molybdenum hook electrode), and a voltage 105applied between electrodes 102 and 103. The direction of gravity isshown by arrow 106. For instance, the carbon nanotube yarn 101 that canbe used are described in the co-owned Li '667 PCT Application.

The applied stress is normalized as a percent F of the room-temperaturetensile strength of precursor yarns, σ_(max), and is designated byITAP-F. Before the ITAP, the highly twisted and the coiled pristinenanotube yarns untwisted and snarled to provide a torque balancedstructure when the yarn ends were not tethered. In contrast, the ITAPcoiled yarns remained straight and negligibly untwisted upon release oftethering. This indicates that the ITAP enhanced inter-nanotubeinteractions, which acted as internal springs to hinder yarn untwist.

FIG. 2A is a top-view scanning electron microscope (SEM) image of apristine twisted yarn. FIG. 2B is a top-view SEM image of an ITAP-40yarn obtained by applying ITAP-40 to the pristine twisted yarn of FIG.2A. A comparison of FIG. 2A and FIG. 2B show that the ITAP-40 yarndecreased both yarn bias angle (a) and diameter (d). The bias angles forthe ITAP yarns were accurately predicted from the bias angles for thepristine yarns and the relative diameters of pristine and ITAP yarns byusing the equation:

α=tan⁻¹(πdT)

where T is the inserted twist per yarn length.

FIGS. 2C-2D are the cross-sectional SEM images of the pristine yarn andthe ITAP-40 yarn of FIGS. 2A-2B, respectively. A comparison of FIG. 2Cand FIG. 2D show the effect of the ITAP-40 on decreasing yarn porosity,which increases from yarn center to yarn surface. Corresponding to theincrease in average yarn density from 0.5 to 0.93 g cm⁻³ as a result ofITAP-40, the percentage of cross-sectional area in the images from voidsdecreased from about 30% to about 15%.

Mechanical test results showed that both the strength and modulusincreased with increasing applied tensile stress during the ITAP andtensile strength and modulus (and specific strength and specificmodulus) substantially increased during ITAP-40 for all investigatedprecursor yarn densities.

The achievable strength and modulus enhancements increased withincreasing mechanical load applied during the ITAP process until amechanical load was applied that resulted in yarn fracture duringmechanical anneal. Since yarn strength increases during thermal anneal,the maximum mechanical load that can be applied during ITAP can beusefully increased by increasing the mechanical load during the ITAPprocess, so that at each moment during this process the applied load isbelow that needed to provide yarn fracture or damage. Hence, a processembodiment is useful wherein the tensile stress applied to the CNT yarnduring thermal anneal increases with increasing time in some timeperiods during the step of high-temperature annealing, while maintainingthis stress at below the applied stress that would cause yarn damage atthe anneal temperature.

The ITAP treatment time can be shortened by increasing the temperatureat which the ITAP treatment is accomplished.

Long-term, high-temperature thermal annealing is known to increase thegraphitization of individual CNTs and improve their mechanicalproperties [Yamamoto 2014]. In contrast, the inventors have found thatapplication of ITAP-30 at ˜2000 C for 10 seconds dramatically improvedthe strengths and moduli of CNT yarns, but did not importantly improvetheir graphitization, as measured by the intensity ratios of thegraphite structure-derived G-band and defect-derived D-band for thetwisted ITAP-30 yarns. Thus, the mechanical property enhancements weremainly attributed to enhanced inter-nanotube connections rather thanindividual nanotube graphitization. Furthermore, the Raman spectra ofthe ITAP-30 yarns annealed for 2 hours did not show the high G/Dintensity ratio of the yarn annealed for the same time but withoutapplying load. These Raman spectroscopy observations suggest that theseenhanced inter-nanotube connections are, at least in part, due tointer-nanotube cross-links, and that these cross-links could contributeto the mechanical property improvement and retention of low G/D ratiosfor the ITAP yarns.

The torque needed to prevent untwist is near zero for the ITAP-40 yarn,since the torque generated by yarn twist is balanced by forces due toITAP-generated inter-nanotube connections. This explains the stabilityof ITAP yarns with respect to untwist.

Chlorosulfonic acid can debundle carbon single wall nanotubes and MWNTsand causes CNT structures to swell and then disintegrate due to itsstrong protonation [Davis 2009; Parra-Vasquez 2010]. The ITAP yarns havelong-term structural and mechanical stability in chlorosulfonic acid,with the nanotubes remaining aligned and densely packed and the yarnsretaining 82% of its modulus and 90% of its strength after immersion inthis acid for 5 minutes. However, the pristine twisted yarn swelled,untwisted, and became disordered after immersion in chlorosulfonic acidfor 4 minutes, which led to a 10-fold decrease in yarn strength and a5.8-fold decrease in modulus. These results suggest that ITAP-inducedcrosslinking prohibited the chlorosulfonic acid from substantiallypenetrating and expanding the ITAP yarns. The ITAP yarns also showedincreased resistance to oxidation in air compared to pristine yarns.

Unless a torsional return spring is provided, previously describedsingle-ply, twist-spun or coiled CNT yarns cannot be used as areversible torsional artificial muscle [Foroughi 2011; Lima 2012; P.Chen 2015]. This problem was first characterized forelectrochemically-driven single-ply, twist-spun muscles [Foroughi 2011].The solution used was to two-end torsionally tether the yarn and toactuate only half of its length, so that the non-actuated lengthfunctioned as a torsional return spring [Foroughi 2011].

However, the liability of this approach is that it decreases the yarnlength that contributes to actuation, and thereby makes the resultingtorsional motors unnecessarily long. Instead of using single-ply coiledyarn, Peng's group utilized a helical thread prepared by coilingmulti-plied straight CNT yarns, which were relatively stable and showedreversible actuation of rotating a lightweight rotor attached at thethread end when driven by solvent infiltration [P. Chen 2015]. Whilesolid guests in previous described hybrid muscles could act as internaltorsional return springs to enable reversible actuation, this restrictsthe type of yarn guest that can be used, thereby eliminating thepossibility of using fully-actuated, non-tethered, single-ply yarns asintelligent actuating sensors that can open and close valves in responseto vapors, liquids, and liquid-delivered important biological materials.For these reasons, previously described tensile muscles for controllingvalves in response to liquid composition or harvesting electrical energyby using liquid waste streams having different compositions were two-endtethered to prevent torsional rotation [Lima 2015].

Fast, reversible torsional and tensile actuation of guest-free ITAPyarns can be simultaneously realized in response to the absorption anddesorption of organic vapors, such as acetone and ethanol. No externaltorsional tethering or external return spring was needed, sinceITAP-produced inter-nanotube connections acted as internal springswithin the ITAP yarn. The actuator simply comprises a one-end-supported,coiled, single-ply ITAP muscle that has attached on its opposite end aheavy rotor. Vapor absorption caused the coiled ITAP yarn to untwist andcontract in length, while vapor desorption made the yarn retwist andincrease in length.

The above major properties changes suggest that the ITAP facilitatescrosslinking of the twisted and the coiled structures by providinglateral stresses that draw nanotubes into close proximity and reduce theenergy barrier for cross-linking. Additionally, ITAP-enhanced nanotubebundling over substantial fractions of the about 200 mm nanotube length(same as the nanotube forest height) can act similarly to cross-links.Previous experimental and simulation results have demonstrated thatnanocarbons such as CNTs, amorphous carbon, and graphene can undergocovalent bond reconfiguration at high temperatures [Terrones 2000; Asaka2011; Colonna 2013; J Huang 2006]. These covalent structure changes,such as inter-nanotube covalent bonding, nanotube coalescence, andformation of graphitic nanoribbons, can be facilitated by the presenceof amorphous carbon and defects in the carbon sidewalls [Gutierrez 2005;Salonen 2002].

In summary, fast, commercially applicable ITAP provides remarkableimprovements in the properties of twist-spun and coiled CNT yarns. Theseimprovements include major increases in yarn strength and modulus,increases in oxidative stability and stability to an acid thatpowerfully protonates yarns and makes them unusable, and the setting ofinserted twist for various applications. Since twist retention duringnanotube weaving is extremely important, especially for the warp yarnsthat are highly strained during weaving, this twist setting can beimportant for commercial production of nanotube textiles for energystorage, harvesting and conversion, sensing, and actuation. This twistretention enables the first single-ply, guest-free, CNT yarns that canserve as reversible tensile and torsional muscles without the need forexternal return springs that degrade performance metrics. The high speedof the ITAP process at high temperatures facilitates the application ofthis process during the continuous fabrication and processing of carbonnanotube yarns, including this that are biscrolled to contain solidguest.

The examples provided herein are to more fully illustrate some of theembodiments of the present invention. It should be appreciated by thoseof skill in the art that the techniques disclosed in the examples whichfollow represent techniques discovered by the Applicant to function wellin the practice of the invention, and thus can be considered toconstitute exemplary modes for its practice. However, those of skill inthe art should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments that are disclosed andstill obtain a like or similar result without departing from the spiritand scope of the invention.

Examples

In the following examples of the application of invention embodiments,spinnable carbon multi-walled nanotube (MWNT) forests were used forpreparing the carbon nanotube yarns. These forests were grown bychemical vapor deposition at about 690 C using iron as catalyst andacetylene gas diluted in argon as the carbon source. A 2-nm-thick irondeposited by electron beam physical vapor deposition was used ascatalyst [M. Zhang 2004].

For the yarns used for mechanical property measurements (before andafter the ITAP), a twist density of 6000 turns m⁻¹ was inserted into a 6to 10 mm wide carbon nanotube aerogel sheet as it was drawn from a MWNTforest. Yarns having a density of about 0.5 g cm⁻³ were spun byinserting twist in a freestanding MWNT sheet that was under nearly zeroapplied tensile stress. A 0.4-mm-diameter stainless steel wire wasplaced at the yarn formation point to apply tensile stress on the yarnduring spinning, which increased yarn density up to 0.8 g cm⁻³. Tofurther increase density, up to ˜1.25 g cm⁻³, increased tension wasapplied to the MWNT sheet as it was spun into yarn by passing the sheetthrough two 0.4-mm-diameter stainless steel wires, and controllingtension by varying the position of these parallel wires. The largediameter yarns that were used to make coiled muscles were prepared byinserting twist into a two-end-tethered, 1.3-cm-wide ribbon comprising a20-layers stack of parallel, forest-drawn MWNT sheets. During ITAP, thestress applied during thermal annealing of the coiled yarn at 2000 C,was just sufficient to avoid yarn snarling at the beginning of thermalanneal. The stress applied during the ITAP process for the twisted yarns(as a percent F of the precursor yarns strength at room temperature,σ_(max)) is designated by ITAP-F.

Example 1: Mechanical Property Enhancement by ITAP

FIG. 3 is a graph showing comparisons of the specific strength 301 andspecific modulus 302 of a pristine yarn (1.08 g cm⁻³ in density) and ofcorresponding ITAP yarns annealed under different applied stresses,where the applied stress during ITAP (G), is normalized to the fracturestrength of the pristine yarn (σ_(max)). The specific strength 301 ismeasured in N tex⁻¹ as shown by the vertical axis 303. The specificmodulus 302 is measured in N tex⁻¹ as shown by the vertical axis 304.

A carbon multi-walled nanotube (MWNT) yarn having a density of 1.08 gcm⁻³ was twist-spun from an about 250-mm-high drawable nanotube forestthat was synthesized by chemical vapor deposition. Transmission electronmicroscopy indicates that these MWNTs contain about 9 graphitic wallsand have a diameter of ˜13 nm. Except as otherwise described, the ITAPinvolved electrically heating in vacuum the yarns that were undervarious applied stresses up to about 2000 C. The pristine MWNT yarn hada specific strength (gravimetric strength) of 0.85 N tex⁻¹ and aspecific modulus (gravimetric Young's modulus) of 43.2 N tex⁻¹.

Annealing this twisted yarn at 2000° C. for 2 minutes without applyingstress (0% applied stress) caused a 10% decrease in strength and a 27%increase in modulus. Both the strength and modulus increased withincreasing applied tensile stress during the ITAP for 2 minutes at 2000°C. At the highest applied stress during ITAP (40% of σ_(max)), theITAP-40 process increased specific strength, specific modulus, anddensity by factors of 1.65, 3, and 1.88, respectively.

Measurements of specific strength and specific modulus as a function ofITAP-30 at 2000° C. indicate that the benefit of this ITAP process onincreasing these mechanical properties was achieved in less than 5minutes and substantially most or all of this benefit can be realizedfor anneal times less than a minute. Excessive annealing times at thistemperature during ITAP (above about 5 minutes) resulted in a decreasein specific strength and specific modulus. FIGS. 4A-4B are graphsshowing, respectively, specific strength (plot 401) and specific modulus(plot 402) as a function of annealing time (at 2000° C. under 30%σ_(max)) during the ITAP. Insets 403 and 405 respectively show thestrength (plot 404) and modulus (plot 406) in N/tex as function of timeduring the first five minutes of annealing. These results demonstratethat near maximum increases in strength and modulus were obtained by theITAP within the first few seconds of annealing.

Independent of the density of the pristine twisted yarn, conductingITAP-40 at 2000° C. for 2 minutes increased the specific modulus andspecific strength, as well as the modulus and strength. Thermalannealing in a furnace provided essentially the same increase in thesemechanical properties as did electrothermal heating by passing a currentthrough the yarn.

Annealing a CNT yarn at higher temperature results in a shorter processtime. When a twisted CNT yarn was treated by ITAP process at ˜2600° C.with 40% of σ_(max) applied stress, only 0.3 s is needed to achievenearly identical mechanical strength and modulus as realized byconducting the ITAP process at 2000° C. for 2 minutes under the sameapplied stress.

FIGS. 5A-5B show that application of ITAP-1.5 to a coiled CNT yarnstabilizes a coiled CNT yarn with respect to both substantial yarnuntwist and yarn snarling when tethering is released. FIGS. 5A-5B are,respectively, SEM images of (a) a pristine coiled MWNT yarn and (b) thesame type of yarn after ITAP-1.5. When not tensionally constrained, thepristine yarn of FIG. 5A relaxed to snarl, whereas the annealed yarn ofFIG. 5B remained straight, and did not undergo untwist. This indicatedthat the ITAP stabilized the twisted and the coiled structures of CNTyarns.

Example 2: ITAP CNT Composite Yarn

CNT/graphene oxide composite yarn was made by infiltrating an aqueoussolution of dispersed graphene oxide particles into CNT sheets duringtwist-spinning. Annealing this CNT/graphene oxide composite yarn at2000° C. with 30% of σ_(max) applied stress for 2 minutes, results in a1.7-fold increase in tensile strength and 4-fold increase of modulus.

Example 3: Torque Reduction by ITAP

The torque needed to prevent untwist is near zero for the ITAP-40 yarn,since the torque generated by yarn twist is balanced by forces due toITAP-generated inter-nanotube connections. However, when tensile stressis applied to the ITAP yarns, this force balance is eliminated, so anexternal torque must be applied to prevent yarn untwist.

FIG. 6 shows the torque needed to counter yarn untwist as a function ofapplied tensile stress for both the pristine yarn and the correspondingITAP-40 yarn, which had a diameter of 33 mm and a bias angle of 36. Thespecific torque (t_(s)) of the pristine yarn and the ITAP yarn are shownby lines 601 and 602, respectively. The ratio of the specific torque(t_(s)) of the pristine yarn to that of the ITAP yarn is shown by line603. The specific torques are measured in N m kg⁻¹ as shown by thevertical axis 604. The ratio of the specific torques are unitless asshown by the vertical axis 605.

For the lowest applied tensile stress (13 MPa), the torque needed toprevent untwist was about 10 times lower for the ITAP yarn than for thepristine twisted yarn. Upon increasing tensile stress up to 260 MPa,this ratio of the torque for the ITAP yarn to that for the pristine yarnbecame about ½.

Example 4: Acid Corrosion Resistivity Enhancement by ITAP

The ITAP yarns have long-term structural and mechanical stability inchlorosulfonic acid, whose strong protonation ability ordinarilydebundles carbon single wall nanotubes and MWNTs and causes CNTstructures to swell and then disintegrate [Davis 2009; Parra-Vasquez2010]. Upon immersion in chlorosulfonic acid for 4 minutes, the pristinetwisted yarn swelled, untwisted, and became disordered, which led to a10-fold decrease in yarn strength and a 5.8-fold decrease in modulus. Incontrast, an ITAP-25 yarn remained aligned and densely packed, did notswell, and retained 82% of its modulus and 90% of its strength afterimmersion in chlorosulfonic acid for 5 minutes.

FIG. 7A is a graph showing comparisons of stress-strain curves forpristine twisted yarns and ITAP-25 twisted yarns before and aftertreatment in chlorosulfonic acid for 5 minutes and subsequent removal ofthis chlosulfonic acid from the yarn. Lines 701-704 are the pristinetwisted yarn (before treatment), acid treated twisted pristine yarn,ITAP-25 twisted yarn (before treatment), and acid treated ITAP-25twisted yarn, respectively. FIG. 7B is a photograph of the pristineyarns 705 and ITAP-25 yarns 706 before immersion in chlorosulfonic acidfor 5 minutes. FIG. 7C is a photograph of the pristine yarns 707 andITAP-25 yarns 708 after immersion in chlorosulfonic acid for 5 minutes.

These results suggest that ITAP-induced crosslinking prohibited thechlorosulfonic acid from substantially penetrating and expanding theITAP yarns.

Example 5: ITAP Yarn as a Torsional and Tensile Actuator

When exposed to acetone vapor, a 24-mm-long, 100-μm-thick coiled ITAPyarn reversibly rotated a 6100 times heavier rotor by 630 (correspondingto a rotation of 26 per millimeter of muscle length). The maximumrotational speed of the rotor was 44 rpm, and the muscle lifted a weightcorresponding to a 2.9 MPa load by about 0.7% of the yarn length.Torsional angle oscillations were observed due to the cyclicinter-conversion of the kinetic energy of the rotating rotor to thestrain energy of rotor rotation as the rotors kinetic energy wasprogressively damped. These oscillations in torsional actuation wereeliminated by operating the muscle at near torsional resonance by usinga vapor on/off cycle frequency of 0.18 Hz. Such resonant operationincreased torsional actuator stroke and maximum rotor speed by factorsof 2.6 and 3.5, respectively (to 52 mm⁻¹ and 160 rpm, respectively). Italso caused a phase shift of about ¼ period between the curves for thetime dependence of torsional and tensile strokes, which provided nearcoincidence of the peaks in rotor speed and tensile stroke.

Reflecting the mechanical robustness of the coiled ITAP yarn toirreversible yarn untwist, reversible torsional and tensile actuationwas obtained even when high weight torsional rotors were deployed. Whileincreasing yarn stress from 2.9 to 13.5 MPa (corresponding to 28,400times the muscle weight) by increasing rotor weight did not dramaticallychange torsional actuation stroke, the corresponding increase of momentof inertia for the rotor (from 8.0×10⁻⁹ to 4.8×10⁻⁷ kg m²) decreasedmaximum rotation speed from 155 to 51 rpm. The obtained maximum torquewas 4.12 N-m per kilogram of the yarn mass, which was several times thetorque of electrochemically and absorption driven CNT muscles [Foroughi2011; P. Chen 2015; He 2015], 50 times the torque generated by themoisture-driven graphene-yarn torsional actuator [Cheng 2014], andcomparable to the static torque of the electrothermally drivenwax-filled CNT muscles [Lima 2012]. Moreover, such ITAP yarns showedhighly reversible torsional actuation.

Additional information of the present invention is included in J. Di etal., “Strong, Twist-Stable Carbon Nanotube Yarns and Muscles by TensionAnnealing at Extreme Temperatures,” Adv Mater 28, 6598-6605 (2016) andthe accompanying J. Di, et al., “Supporting Information for Strong,Twist-Stable Carbon Nanotube Yarns and Muscles by Tension Annealing atExtreme Temperatures,” which both are hereby incorporated herein byreference.

While embodiments of the invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the invention. Theembodiments described and the examples provided herein are exemplaryonly, and are not intended to be limiting. Many variations andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, other embodiments arewithin the scope of the following claims. The scope of protection is notlimited by the description set out above.

The disclosures of all patents, patent applications, and publicationscited herein are hereby incorporated herein by reference in theirentirety, to the extent that they provide exemplary, procedural, orother details supplementary to those set forth herein.

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What is claimed is:
 1. A process comprising: (a) applying a tensilestress to a CNT yarn; and (b) high-temperature annealing the CNT yarnwhile the tensile stress is applied to the CNT yarn to form an ITAPyarn, wherein the high-temperature annealing is performed in the rangebetween 1000 C and 3000 C, and wherein the ITAP yarn has acharacteristic selected from the group consisting of (i) the tensilestrength of the ITAP yarn is greater than the pristine CNT yarn, (ii)the tensile modulus of the ITAP yarn is greater than the pristine CNTyarn, (iii) the pristine CNT yarn was a twisted or coiled CNT yarn, andthe twisted or coiled ITAP yarn is stabilized with respect toirreversible untwist or snarling, thereby avoiding the need to tetherthe twisted or coiled ITAP yarn, (iv) the ITAP yarn is stabilized withrespect to chemically-induced yarn degradation, and (v) combinationsthereof.
 2. The process of claim 1, wherein the step of high-temperatureannealing comprises heating the yarn by a method selected from the groupconsisting of: (a) applying an electrical current through the CNT yarn,(b) placing the yarn in a high-temperature environment, (c) absorptionof electromagnetic radiation, (d) inductive heating, and (e)combinations thereof.
 3. The process of claim 1, wherein the CNT yarn isnot coiled.
 4. The process of claim 1, wherein (a) the CNT yarn istwisted and (b) the tensile stress applied to the CNT yarn during thestep of high-temperature annealing is at least 5% of fracture strengthof the CNT yarn at room temperature before the step of high-temperatureannealing.
 5. The process of claim 4, wherein the tensile stress appliedto the CNT yarn during the step of high-temperature annealing is atleast 20% of the fracture strength of the CNT yarn at room temperaturebefore the step of high-temperature annealing.
 6. The process of claim5, wherein the tensile stress applied to the CNT yarn during step ofhigh-temperature annealing increases with increasing time over a firsttime period occurring during the step of high-temperature annealing,while maintaining the tensile stress at below an applied stress levelthat would cause damage to the CNT yarn at the high-temperatureannealing temperature.
 7. The process of claim 1, wherein (a) the CNTyarn is coiled and mandrel-free, and (b) the tensile stress applied tothe CNT yarn is in an amount that avoids yarn snarling of the CNT yarn.8. The process of claim 7, wherein the tensile stress applied to thecoiled CNT yarn is at least 1% of fracture strength of the coiled CNTyarn at room temperature before the step of high-temperature annealing.9. The process of claim 1, wherein an inert environment is employedduring the step of high-temperature annealing.
 10. The process of claim1 further comprising removing oxygen adsorbed from the CNT yarn beforethe CNT yarn reaches incandescent temperatures during annealing.
 11. Theprocess of claim 10, wherein the step of removing oxygen comprises amethod selected from the group consisting of: (a) applying an electricalcurrent through the CNT yarn, (b) placing the yarn in a high-temperatureenvironment, (c) absorption of electromagnetic radiation, (d) inductiveheating, and (e) combinations thereof.
 12. The process of claim 1,wherein time of the step of the high-temperature annealing ranges from0.1 milliseconds to 2 hours.
 13. The process of claim 1 furthercomprising the method of forming the CNT yarn, wherein the step offorming the CNT yarn is selected from the group consisting of spinningfrom CNT forests, CNT solutions, and CNT aerogel sheets grown byfloating catalytic chemical vapor deposition.
 14. The process of claim1, wherein the process is a continuous process or a batch by batchprocess.
 15. The process of claim 1, wherein the CNT yarn is part of anassembly of CNT yarns.
 16. The process of claim 15, wherein the CNTyarns in the assembly are held under different levels of tensilestresses and temperatures.
 17. The process of claim 15, wherein not allof the CNT yarns in the assembly are subjected to the process ofclaim
 1. 18. The process of claim 15, wherein all of the CNT yarns inthe assembly are substantially subjected to the same stress andtemperature.
 19. The process of claim 15, wherein the CNT yarns arewoven into a textile.
 20. The process of claim 15, wherein at least someportion of the CNT yarns are plied.
 21. The process of claim 1, whereinsegments of the CNT yarn are subjected to the process of claim 1 andother segments of the CNT yarn are not subjected to the process ofclaim
 1. 22. The process of claim 1, wherein the CNT yarn furthercomprises at least one additional material other than CNTs.
 23. Theprocess of claim 22, wherein the CNT yarn comprises a ceramic material.24. The process of claim 22, wherein the CNT yarn comprises a secondcarbon material, wherein the second carbon material is not CNTs.
 25. Theprocess of claim 24, wherein the second carbon material comprisesgraphene or a graphene derivative.
 26. The process of claim 1, whereinthe CNT yarn substantially comprises only CNTs.
 27. A coiled or highlytwisted CNT yarn that substantially contains twist in only one directionand comprises substantially only CNTs, wherein the CNT yarn has acharacteristic selected from the group consisting of: (i) the CNT yarndoes not undergo snarling when untethered, (ii) the CNT yarnsubstantially retains twist during the release of tethering even whensnarling is prohibited, (iii) the CNT yarn substantially retainsmechanical strength when exposed to chlorosulfonic acid for 5 minutes atambient temperature, and (iv) combinations thereof.
 28. The coiled orhighly twisted CNT yarn of claim 27, wherein the CNT yarn comprises eachof the following characteristics: (i) the CNT yarn does not undergosnarling when untethered, (ii) the CNT yarn substantially retains twistduring the release of tethering even when snarling is prohibited, and(iii) the CNT yarn substantially retains mechanical strength whenexposed to chlorosulfonic acid for 5 minutes at ambient temperature. 29.An artificial muscle, composite structure, or textile comprising one ormore coiled or highly twisted CNT yarns made by a process comprising:(a) applying a tensile stress to a CNT yarn that substantially comprisesonly CNTs and (b) high-temperature annealing the CNT yarn while atensile stress is applied to the CNT yarn to form an ITAP yarn, whereinthe high-temperature annealing is performed in the range between 1000 Cand 3000 C.
 30. The artificial muscle, composite structure or textile ofclaim 29, comprising one or more twisted CNT yarns wherein the tensilestress applied to the twisted CNT yarns during the step ofhigh-temperature annealing is at least 10% of fracture strength of theCNT yarn at room temperature before the step of high-temperatureannealing.
 31. The artificial muscle, composite structure or textile ofclaim 29, comprising one or more coiled CNT yarns wherein the tensilestress applied to the coiled CNT yarns during the step ofhigh-temperature annealing is at least 1% of the fracture strength ofthe coiled CNT yarn at room temperature before the step ofhigh-temperature annealing.